1. Field of the Invention
The present invention generally relates to variable gain amplifier circuits, and particularly to variable gain amplifier circuits adjusting the level of audio signal by varying the gain of the signal based on a control signal.
2. Description of the Related Art
As an exemplary variable gain amplifier circuit, there is known a variable gain amplifier circuit adjusting the level of an audio signal by varying the gain of the signal based on a control signal corresponding to the peak-to-peak level of the audio signal captured from various audio sources.
FIG. 6 is a schematic circuit diagram of a conventional variable gain amplifier circuit integrated into a semiconductor chip. As shown in FIG. 6, an audio signal from various audio sources is input into a terminal 1, and the input audio signal is supplied to each non-inverting input terminal of operational amplifiers 2, 4, 6, 8.
As shown in FIG. 6, the output terminal and the inverting input terminal of the operational amplifier 2 are connected to each other via a resistor R1. The inverting input terminal is connected to one end of resistor R2. A referential voltage (Vref) is applied to the other end of the resistor R2. The output terminal of the operational amplifier 2 is connected to a terminal 10. As a result, the operational amplifier 2 is configured as a non-inverting amplifier. The operational amplifier 2 operates only when, for example, a high-level control signal is supplied to the terminal 3 of the operational amplifier 2. The amplification of the non-inverting amplifier is determined by R1 and R2 and is obtained by the formula (=1+R1/R2). In this example, the amplification of the non-inverting amplifier is 6 dB (2 times).
The output terminal and the inverting input terminal of the operational amplifier 4 are connected to each other via a resistor R3. The inverting input terminal is connected to one end of resistor R4. The referential voltage (Vref) is applied to the other end of the resistor R4. The output terminal of the operational amplifier 4 is connected to a terminal 10. As a result, the operational amplifier 4 is configured as a non-inverting amplifier. The operational amplifier 4 operates only when, for example, a high-level control signal is supplied to the terminal 5 of the operational amplifier 4. The amplification of the non-inverting amplifier is determined by R3 and R4 and is obtained by the formula (=1+R3/R4). In this example, the amplification of the non-inverting amplifier is 4 dB (1.58 times).
The output terminal and the inverting input terminal of the operational amplifier 6 are connected to each other via a resistor R5. The inverting input terminal is connected to one end of resistor R6. The referential voltage (Vref) is applied to the other end of the resistor R6. The output terminal of the operational amplifier 4 is connected to a terminal 10. As a result, the operational amplifier 6 is configured as a non-inverting amplifier. The operational amplifier 6 operates only when, for example, a high-level control signal is supplied to the terminal 7 of the operational amplifier 6. The amplification of the non-inverting amplifier is determined by R5 and R6 and is obtained by the formula (=1+R5/R6). For example, the amplification of the non-inverting amplifier is 2 dB (1.26 times).
The output terminal and the inverting input terminal of the operational amplifier 8 are connected to each other. The output terminal of the operational amplifier 8 is connected to the terminal 10. As a result, the operational amplifier 8 is configured as a buffer amplifier. The operational amplifier 8 operates only when, for example, a high-level control signal is supplied to the terminal 9 of the operational amplifier B. The amplification of the buffer amplifier is 0 dB (1 time).
Only one of the control signals supplied to the terminals 3, 5, 7, 9 is high-level. Accordingly, only one of the operational amplifiers 2, 4, 6, 8 is to be operated and an audio signal amplified by the one of the operational amplifiers 2, 4, 6, 8 is output from the terminal 10.
FIG. 7 is a schematic circuit diagram of an exemplary non-inverting amplifier of the operational amplifier 2. The configuration of the other non-inverting amplifiers of the operational amplifiers 4, 6, 8 is substantially the same as the configuration of the operational amplifier 2. In FIG. 7, the emitters of npn transistors Q1, Q2 are connected to each other and are grounded via a constant current source 11 and a switch 12. The base of the transistor Q1 is connected to a terminal 1, and the collector of the transistor Q1 is connected to voltage Vcc via a constant current source 13. The base of the transistor Q2 is connected to the junction of one end of resistor R1 and one end of resistor R2. The collector of the transistor Q2 is connected to the voltage Vcc. As a result, a differential circuit is configured with the transistors Q1 and Q2.
The collector of the transistor Q1, that is the output of the differential circuit, is connected to the base of a pnp transistor Q3. The emitter of the transistor Q3 is connected to the voltage Vcc. The collector of the transistor Q3 is grounded via a constant current source 14 and a switch 15. As a result, the transistor Q3 operates as an output circuit with the emitter grounded. The collector of the transistor Q3 is connected to the terminal 10, the base of the transistor Q2 via the resistor R1, and the base of the transistor Q3 via a capacitor C0 for phase compensation.
The reference voltage is applied to the base of the transistor Q2 via the resistor R2. Switches 12, 15 are closed only when high-level control signal is applied to the terminal 3 to flow current through transistors Q1 through Q3.
Japanese Utility Model Publication Application No. H4-102311 discloses an amplifier circuit in which gain of the amplifier circuit is determined by selecting only one of two differential amplifier circuits in the amplifier circuit so as to apply power to the selected differential amplifier only.
FIG. 6 is a schematic circuit diagram showing an example of a conventional variable gain amplifier circuit. As shown in FIG. 6, the resistors R1 through R6 are necessary to be provided to set the amplification of each non-inverting amplifier. Further, as shown in FIG. 7, an output circuit including a transistor, a constant-current source, and a switch is necessary for each non-inverting amplifier. Unfortunately, because of the structure, the number of circuit elements of the above conventional variable gain amplifier circuit is large and, accordingly, the area of semiconductor integrated circuits becomes large to integrate the circuit elements in the semiconductor integrated circuit.